Synaptic connections in the nervous system are highly specific. In some regions of the central nervous system, synapse specificity arises from the reorganization of more diffuse, early patterns of connectivity whereas in other regions, precise patterns may be present from the very beginning. Thus, one of the most challenging and important issues in neurobiology concerns how neuronal circuits are established with precision during development. We are interested in understanding how neural circuits are formed and organized in the vertebrate retina, and in particular how interactions between potential pre- and postsynaptic cells guide this process during development. Although much knowledge has been gained from in vitro work, it is evident that examining this process in vivo will provide insight into the dynamic interactions that take place to establish synaptic specificity. The zebrafish is an ideal model for studying the in vivo development of retinal circuits. This is because synapse formation is completed within a few days after fertilization and the zebrafish embryo can be maintained transparent, making it suitable for in vivo imaging throughout the period of synapse formation and maturation. In this application, we propose to focus on the development of networks in the inner retina. We will determine how outgrowth and elaboration of the postsynaptic dendrites of retinal ganglion cells and the presynaptic terminals of amacrine cells, that together form the first retinal network, contact and form the synaptic region, the inner plexiform layer (IPL) during development. We will combine time-lapse in vivo imaging techniques with molecular approaches to elucidate the normal pattern of IPL development, and then apply these techniques together with the use of retinal mutants to ascertain the role of cell-cell interactions in organizing this synaptic layer. Together, the results of the proposed studies will further our understanding of the mechanisms underlying the structural and functional development of the inner retinal circuitry.